The trend is likely to worsen as water temperatures continue to rise, the scientists say.

While the species reaching European beaches aren't considered lethal to humans, some like the purple jellyfish have a very painful sting that can cause severe swelling and an allergic reaction.

The Spanish Red Cross is reported to have treated more than 19,000 bathers for jellyfish stings in the famous Costa Brava resort region alone.

Government officials have sent out boats to net the jellyfish before they reach shallow water, and many Spanish beaches have been closed.

Resorts in Italy and France have also been badly affected.

Red flags and signs have appeared on beaches across the western Mediterranean to warn vacationers of the danger.

Last month researchers from the marine environmental group Oceana Europe, based in Madrid, Spain, discovered massive concentrations of jellyfish along Spain's southeast coast.

"We have found jellyfish all over the Mediterranean, but in this area we've seen concentrations of more than ten jellyfish per square meter [11 square feet]," reported head of research Ricardo Aguilar.